1. Field of the invention
This invention refers to a system to increase the conversion yield and the reduce the energy consumptions in the reactors of heterogeneous synthesis reactors, in particular ammonia reactors, consisting of an external shell (P), usually in one piece, and of an internal cartridge (C) having a diameter (Dc) and containing granular catalyst arranged in one or more catalytic beds (K) that have an axial height (Hi) and that are closed between one external cylindrical wall made up of sections of said cartridge (C) having an internal diameter (Dc) and a height (Hi), an internal cylindrical wall made up of sections of an internal feed pipe for the quench gas (T) having a diameter (Dt) (small with respect to Dc) and a height (Hi), the upper transversal face of each bed being open and invested axially by synthesis gas.
As already known, the reactors for catalytic synthesis under pressure, particularly for the catalytic synthesis of ammonia, methanol, higher alcohols, etc., consist of an external shell, generally in one piece and an inner cartrdige shell containing a granular catalyst arranged in one or more layers (catalytic beds).
The synthesis gas that crosses the various catalytic beds is generally cooled between one bed and the other, with the aim of obtaining optimal temperature conditions in the various beds by means of the injection of fresh gas (quench reactor) or by means of a direct interchange with cold inlet gas.
2. Description of the Prior Art
Recently reactors have been proposed with gas radial flow in the catalytic beds (Lummus, Topsoe, Kellogg, U.S. Pat. Nos. 3,918,918 and 4,181,701, European Patent Application no. 007,743-A1) or with axial-radial flow (Ammonia Casale U.S. Pat. Nos. 4,372,920 and 4,405,562) that represent an important progress in comparision to reactors with axial flow, particularly when a large volume of catalyst must be exploited, permitting the reduction of the charge loss reduced across the catalytic beds, and therefore, of the energy consumptions. The axial flow of the gas in these cases means the adoption of reactors developed in width (low length-diameter ratio of the equipment) with high costs of the machinery and high energy consumption.
The ammonia Casale U.S. Pat. Nos. 4,372,920 and 4,405,562 have allowed a considerable simplification of the reactor's cartridge design favouring the obtainment of an internal reactor structure, with easy access for the maintenance and the loading and the substitution of the catalyst, and at the same time with low charge losses. According to the above patents, each catalytic bed is made up of a perforated external cylindrical wall, of a perforated internal cylindrical wall and of only one closed bottom (the upper wall of the basket is in fact completely open); an unperforated upper portion of the above internal cylindrical wall (or of both the cylindrical walls), in continuation with the open cross section between the upper edges of the two cylindrical walls arranged on a plane approximately perpendicular to the longitudinal axis of said perforated walls, forms a passage where a minor portion of the gas crosses the bed with a prevailingly axial flow, whereas the remaining major portion of the gas crosses with radial flow the major cross section of the catalytic bed arranged in the perforated zone of the two cylindrical bed walls.
According to the patented invention, the gas portion that crosses the bed with prevailingly axial flow is controled by the height of the portion of the unperforated internal (or internal or external) cylindrical wall, that is in all cases, a minor portion of the total height of the said wall. Reactors with dimensions corresponding to a height-diameter ratio higher than 10 (f.i. with diameter-height ratio lower than 0.1) are preferably realized according to the said patented invention.
In the present world-wide economic situation, the market for the modernization of existing plants is very important, the major part of which especially, in the case of synthesis processes (f.i. ammonia synthesis) use reactors with axial gas flow in the catalytic beds (axial reactors), characterized by a low height-diameter ratio of the equipment, due to the above mentioned necessity to reduce the charge loss within the machinery.
In preceding patent applications, particularly in the European patent application 86,104,911.2, the Applicants have already described a system to reduce energy consumptions of heterogeneous catalytic synthesis reactors under pressure, in particular of reactors with dimensions corresponding to low height-diameter ratios (lower than 10); in particular a system is described which is an ideal system to be utilized in the modernization of existing reactors with axial gas flow, in particular in Kellogg-type reactors which are used in numerous large-scale plants (800.div.1500 MTD) mostly built towards the end of the 1970's. The main characteristics of the said plants, (besides the high capacity already cited), is the use of centrifugal machinery for the compression of gas operated with vapour produced in the plant itself, according to an integrated cycle between the process vapour and the vapour used to operate the machinery, and the use of the said axial Kellogg-type reactors.
The said plants are also characterized by the uncritically high energy consumptions of those years in which energy was available at a low cost, the axial-type reactor being the cause of said high consumptions.
In the continuation of their researches the Applicants have found a new system that is now also easily applied for the modernization of "Chemico"-type reactors.